Build an Adirondack chair By Les Kenny

Step 5. Add the bottom rail

In this context, the bottom rail is a piece shaped specifically to give the back boards their curved profile.
The bottom of the back boards are fixed to the bottom rail.
The bottom rail is fixed on top of the rear spreader.

Apply some glue and center the bottom rail top of the rear spreader, with the back edge of the bottom rail
flush with the back of the rear spreader.